1. Field of the Invention
The present invention relates to a planar commutator, a rotor and a direct current electric motor.
2. Description of Related Art
An axial-gap motor, which includes a planar rotor (having a planar commutator) and a planer stator assembled together to oppose each other in a direction of a rotational axis of the planar rotor, is known (see, for example, Japanese Unexamined Patent Publication No. 2004-274971 or PCT Publication No. WO 03/047069 corresponding to US Patent Publication No. 2005/0017596A1).
In a case of a brush electric motor, which has two or more pole pairs (multiple poles), a spark may possibly be generated at a commutator due to voltage unbalance between brushes, which are supposed to have the same electric potential. In order to limit wearing of the brushes caused by the spark or to reduce the number of the brushes, it is known to use equalizing lines, i.e., short-circuit lines (equalizing devices), such as copper wires or copper pieces, to interconnect between corresponding segment pieces of the commutator, which have the same electric potential.
Furthermore, at the time of interconnecting the segment pieces with the equalizing lines, the equalizing lines may possibly contact with each other to cause short-circuiting therebetween or may contact with an armature to cause short-circuiting therewith. Also, at the time of interconnecting the segment pieces with the equalizing lines, the winding operation (fusing operation) may possibly become tedious. Japanese Unexamined Patent Publication No. H11-187622 discloses a technique that addresses the above potential disadvantages. According to this technique, each pair of segment pieces is interconnected by an arcuate short-circuit element on a rear side thereof (a rotational axis side). Then, all of the pairs of segment pieces are placed along a circle, and a dielectric resin material is filled radially inward of the segment pieces to form the commutator.
When the short-circuit elements are placed radially inward of the commutator, the short-circuiting between the equalizing lines or between the equalizing lines and the armature can be limited, and it is not required to perform the tedious winding operation for winding the equalizing lines in the slot of the commutator.
Japanese Unexamined Patent Publication No. 2000-60073 (corresponding to U.S. Pat. No. 6,057,623) discloses another technique. According to this technique, an equalizing device (equalizer) is installed to a rotatable shaft at a location adjacent to the commutator. In the equalizing device, a plurality of annular terminals is stacked over a base member such that a dielectric insulator is interposed between each adjacent two annular terminals. Two equalizing lines, which project radially outward from each terminal, are electrically connected to corresponding two segment pieces by fusing. In this way, the segment pieces, which need to have the same electric potential, are electrically short-circuited. However, in the above technique of Japanese Unexamined Patent Publication No. H11-187622 for integrally forming the commutator and the short-circuit elements by placing the short-circuit elements in the interior of the commutator, the placement of the short-circuit elements in the interior of the commutator may become difficult when the diameter of the commutator becomes small. Also, in the above technique of Japanese Unexamined Patent Publication No. 2000-60073 (corresponding to U.S. Pat. No. 6,057,623) for placing the equalizing device coaxially with the commutator, the segment pieces need to be interconnected by the equalizing lines of the equalizing device by, for example, fusing, thereby requiring the tedious work that causes a reduced work efficiency.
In order to address the above disadvantages, Japanese Unexamined Patent Publication No. 2004-222364 discloses another technique. According to this technique, an end surface dielectric insulator covers an axial end surface of an armature. A plurality of pairs of terminals is provided in the dielectric insulator. Each pair of terminals, which are electrically interconnected with each other, is connected to a corresponding pair of segments pieces through connecting parts of the commutator. With this technique, the connecting operation for connecting the commutator with the short-circuit elements is eased and thereby improving the work efficiency while not being limited by the diameter of the tubular commutator.
However, in a case where the above tubular commutator is applied to the planar motor, there are disadvantages in terms of the axial size, the current density and the slide loss between the brushes and the commutator. Therefore, in the axial-gap electric motor, a planar commutator is often used. In such a case, brushes are engaged with the planar commutator in the axial direction of the axial-gap electric motor and slide over the planar commutator along a plane. In the case of the planar commutator, the equalizing elements are placed below the commutator. Thus, in the case of the motor, which includes the commutator, the short-circuit elements and the planar armature, the thinning of the motor is limited.